﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

namespace Infrastructure {
    /// <summary>
    /// 表达式组合器
    /// </summary>
    public static class ExpressionCombiner {
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) {
            return Compose(first, second, Expression.AndAlso);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) {
            return Compose(first, second, Expression.OrElse);
        }

        private static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge) {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression 
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        partial class ParameterRebinder : ExpressionVisitor {
            private readonly Dictionary<ParameterExpression, ParameterExpression> map;

            public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map) {
                this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }

            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp) {
                return new ParameterRebinder(map).Visit(exp);
            }

            protected override Expression VisitParameter(ParameterExpression p) {
                ParameterExpression replacement;
                if (map.TryGetValue(p, out replacement)) {
                    p = replacement;
                }
                return base.VisitParameter(p);
            }
        }
    }
}